1. Field of the Invention
The present invention is related to accelerometers, and more particularly, to servo-compensated pendulum-type linear accelerometers with flexible suspension of the sensing element.
2. Background Art
A conventional accelerometer is described in U.S. Pat. No. 4,498,342. Such an accelerometer includes a housing, a proof mass (“PM,” sometimes also called “pendulum inertial mass”) that is placed in a gas damping chamber, and is flexibly attached to a ring frame, which is itself placed in the housing. A differential angular sensing element, a preamplifier, a correcting element, an amplifier, and a momentum sensor with moving coils, all collectively form a feedback circuit of such an accelerometer. The proof mass, the flexible suspension, and the frame are all manufactured from a single silicon monocrystalline wafer.
In this accelerometer, the momentum sensor includes two permanent magnets with cup-like magnetic conductors and two balancing coils, which are located on both sides of the wafer PM. Current that flows through each coil creates a magnetic field, which interacts with the magnetic field of the corresponding magnetic stator, forming a compensating force which is applied to the proof mass. By controlling the electric current that passes through the coils, it is possible to control the magnitude and direction of the compensating force. The differential angular sensor of this device uses a bridge circuit, that includes strain resistors. These strain resistors are formed on the silicon elements of the elastic suspension. The motion of the proof mass relative to the housing leads to a change in the differential resistance of the angular sensing element. This change in the resistance is used to determine the position of the proof mass.
When in use, the accelerometer is normally affixed to the object whose acceleration needs to be measured. The acceleration of the object along the sensitivity axis of the accelerometer leads to the rotation of the proof mass around the axis of suspension, relative to the stators of the momentum sensor. The change in the differential resistance, caused by the movement of the proof mass, is sensed by the feedback circuit. The feedback circuit reacts, generating a current, which is supplied to the balancing coils. As a result, a compensating force is generated, which returns the proof mass to its neutral position. The amount of current required to return the proof mass to its neutral position permits measuring the acceleration along the axis of sensitivity of the device.
This conventional accelerometer has a number of disadvantages. The use of a bridge circuit with strain resistors leads to a relatively large temperature drift of the zero bias of the accelerometer. Since the proof mass has elements of flexible suspension, the zero bias drift causes a fairly significant error in the compensating accelerometer's output, with the error being proportional to the stiffness of the flexible suspension.
Another problem with the conventional accelerometer described above is the lack of a system of adjustment of the zero bias drift for the signal that corresponds to angular displacement of the proof mass.
Yet another problem with this conventional accelerometer is the use of silicon, which makes the accelerometer more vulnerable to shock and vibration, particularly when the accelerometer is turned off.
Another conventional accelerometer is described in U.S. Pat. No. 6,073,490. This accelerometer includes a housing that can be disassembled, and a proof mass, which is connected to a frame using flexible suspension that is itself mounted within the housing. Both halves of the housing are in the shape of rectangular bars with cylindrical openings. A cup-like magnetic circuit, with the magnetic circuits having a gap between the field concentrators. The magnetic circuit also has a central core that includes a permanent magnet and a field concentrator. Both rectangular bars of the housing are formed of monocrystalline silicon, where the axes of the silicon crystal have the same orientation as the proof mass. The permanent magnet also has toroidal coils mounted thereon, with the toroidal coils being used to measure angular displacement, including differential measurement of the angular displacement.
The left and right halves of the accelerometer's housing are connected together by elastic elements (such as springs). Such an accelerometer functions as a fairly conventional device of the servo compensating type. When the base of the device moves with an acceleration that is directed along the axis of sensitivity of the device (“SA” in the figures of the U.S. Pat. No. 6,073,490 patent), the proof mass is displaced due to the momentum caused by the acceleration being measured. The angular displacement sensor converts the angular displacement of the proof mass into an electric signal. A preamplifier, a modulator, a correcting element, and an amplifier, all connected in series, form a servoamplifier of the accelerometer, and provide a signal that is supplied to the momentum sensor, and which is used to balance the momentum. The current that flows through the coils of the momentum sensor is the output signal of the accelerometer.
The accelerometer described above is particularly interesting for its mechanical construction and the principle on which the angular displacement sensor works. The moveable coils of the momentum sensor receive an AC signal from a high frequency generator, which is connected to the input of the amplifier. This current creates an AC magnetic flux, which passes through the toroidal coils of the angular displacement sensor, which are located on the central cores of the magnetic circuits of the momentum sensor. This magnetic flux, which passes through the toroidal coils of the angular displacement sensor, changes when the position of the proof mass relative to the housing is changed, due to the fact that the displacement of the proof mass changes the magnetic coupling between the moveable coils and the magnetic circuit.
Alternating voltages are induced in the coils of the angular displacement sensor. The amplitude of these voltages depends on the mutual position of the moveable coils of the momentum sensor and the coils of the angular displacement sensor, in other words, on the position of the proof mass of the accelerometer relative to its housing. The differentially coupled coils of the angular displacement sensor create a high frequency AC output signal. Its amplitude is proportional to the angular displacement of the proof mass, while its phase depends on the direction of the angular displacement of the proof mass. Thus, in this accelerometer, movable coils of the momentum sensor act as primary windings of the angular displacement sensor. Both the output coils of the angular displacement sensor and the coils of the momentum sensor use a common magnetic circuit. This simplifies the construction of the accelerometer.
The accelerometer of U.S. Pat. No. 6,073,490 has a number of disadvantages. One of the disadvantages is the fact that the proof mass and its flexible suspension and its frame are both manufactured from monocrystalline silicon, while the magnetic circuits are manufactured from metallic elements. The coefficients of thermal expansion of the various elements of the various parts of the accelerometer are different. This leads to a temperature-dependent zero bias drift, which frequently requires selection of particular combinations of materials from which the various parts of the accelerometer are made.
Another disadvantage is that the construction of the device includes a means for electrically tuning the output zero bias drift signal of the angular displacement sensor. Such a tuning, is generally accomplished by short circuiting some portions of the toroidal coils that are located around the central cores of the magnetic circuit.
As shown in FIG. 4 of U.S. Pat. No. 6,073,490, it is possible to mechanically tune the output zero bias drift signal of the angular displacement sensor. This is done by changing the position of the ferromagnetic core relative to the angular displacement sensor coils. However, it should be remembered that the permanent magnets are on the outside, while the metallic magnetic conductor, within which the core moves, is on the inside. This leads to external magnetic fields affecting the magnetic flux within the accelerometer, which reduces the accuracy of the measurement.
Another disadvantage is that the device has a single base surface on which it rests, which is perpendicular to its axis of sensitivity. When measuring two or three projections of the acceleration vector of the object onto which the accelerometer has been affixed, the existence of only a single base mounting surface complicates the process of installation of the accelerometer, and requires additional mounting elements and fixtures for mounting the accelerometer.
The use of moving coils in the accelerometer, in the momentum rotation sensor, as excitation coils of the angular displacement sensor, reduces the magnitude of the maximum allowed voltage at the output of the amplifier by the amplitude of the high frequency excitation, as discussed above.
Accordingly, there is a need in the art for an accelerometer that addresses at least some of the above disadvantages.